The p21^ras (Ras) family of signal switch proteins is deregulated in myeloid malignancies by genetic mechanisms that include NRAS and KRAS2 point mutations, the BCR-ABL fusion, PTPN11 mutations, and NF1 inactivation. We developed accurate mouse models of myeloproliferative disease (MPD) by exploiting the Mx1-Cre transgene to ablate a conditional mutant Nf1 allele or to activate oncogenic Kras^G12D expression from its endogenous promoter (1, 2). In recent work, we exploited this strategy to induce endogenous oncogenic Nras^G12D expression in hematopoietic cells, and unexpectedly observed marked phenotypic differences in Kras and Nras mutant mice. Studies in this new model formed the basis of a successful application for supplemental funding support through the ARRA to extend the scope of this R37 award to investigate leukemogenesis in Nras mice. We have made extensive use of retroviral insertional mutagenesis to induce progression from MPD to acute myeloid leukemia (AML) and T lineage acute lymphoblastic leukemia (T-ALL) in Nf1, Kras, and Nras mutant mice, and we are harnessing these aggressive and genetically heterogeneous cancers to investigate mechanisms of drug response and resistance in vivo.